This invention relates to a thread roll-forming or thread-rolling apparatus particularly adapted to provide tapered screw threads on, for example, gas pipes and pipe fittings.
In one conventional method of forming tapered screw threads on the end portion of a pipe such as a gas pipe, the screw threads are formed by means of threading dies, which cut the threads and remove material. The inner diameter of the pipe thus machined remains constant, whereby the pipe wall thickness decreases progressively toward the extremity of the pipe, the strength of which at its end portion is thereby lowered.
In another conventional method, rolling dies are pressed toward the end portion of a pipe to form rolled threads without cutting the pipe, whereby the pipe material is not damaged, and the effective strength of the screw threads is preserved. However, this conventional method entails a number of difficulties as will be described more fully herinafter.